The present invention relates to a focusing device for a camera equipped with both a standard lens and a telephoto lens which can be arbitrarily selected for exposure framing.
The so-called pocket cameras are very popular mainly because of their simple operations or manipulations, that is, it is easy to load a casette of film or the like and no focusing is needed. In addition to the pocket cameras with only a standard lens, those equipped with a standard lens and a telephoto lens or a wide-angle lens as well have been put into the market recently. With such a camera, a user can arbitrarily select a standard lens or a telephoto or wide-angle lens depending upon a desired frame.
It is a well known fact that in general the shorter the focal length of a camera lens, the deeper the depth of field becomes.
A standard lens built into a pocket camera is such that it can sharply focus a subject at a relatively closer distance to a subject at infinity; that is, the standard lens has a deep depth of field. A wide-angle lens has, of course, a focal length shorter than that of a standard lens. Therefore, with a pocket camera equipped both with a standard lens and a wide-angle lens, the selection and use of either of the standard or wide-angle lens will not cause any adverse effects on focusing. However, a pocket camera equipped with both a standard lens and a telephoto lens gives rise to some problems. Since the telephoto lens has a long focal length, its depth of field is shallow. Therefore, in the case of the photography or exposure with the telephoto lens, a subject must be sharply focused. Otherwise, it would be out of focus.